"What does Death the Kid's mom look like, and why she doesn't appear in the series?"

Well, here's how my image on what does Lady Death might look like.

I pictured her looking like a mix between Mortisha Addams (The Addams Family) and Elvira, and imagined her being a immortal human (since I think that Kid is a demi-god child, and that explains why he looks more human than his father). And I named her "Evangeline," cause I thought it suits her.

As for the reason for her not being revealed, I'm guessing that she's one of those women guys call a "MILF".And Lord Death is protecting her from perverted men, especially Spirit. .Plus, Kid would feel uncomfortable if Soul and Black Star find her attractive.

And for a bonus, I drew Lady Death with a baby version of Kid.^^

Soul Eater series belong to OkuboLady Evangeline Death designed by me.